Electrophotographic marking is a well-known, commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a charged photoreceptor with a light image representation of a desired document. The photoreceptor is discharged in response to that light image, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image, which is then transferred onto a substrate, such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a permanent record of the original representation. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of other images.
One way of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is typically comprised of a laser light source (or sources), a pre-polygon optical system, a rotating polygon having a plurality of mirrored facets, and a post-polygon optical system. In a simplified description of operation a collimated light beam is reflected from facets of an optical polygon and passes through imaging elements that project it into a finely focused spot of light on the photoreceptor surface. As the polygon rotates, the focused spot traces a path on the photoreceptor surface referred to as a scan line. By moving the photoreceptor as the polygon rotates the spot raster scans the surface of the photoreceptor. By modulating the laser beam with image information a predetermined latent image is produced on the photoreceptor. The plane of the sweeping beam is referred to herein as the tangential optical plane while the direction of motion of the photoreceptor is in the sagittal direction.
Referring now to FIG. 1, a typical prior art imaging system 6 includes a laser diode 8 that emits a laser beam 10 that is modulated in response to drive signals from a controller 12. When emitted from laser diode 8, the laser beam 10 is divergent. Lens 14 collimates that diverging beam and the collimated output is directed through cylindrical lens 16 that has focusing power only in the sagittal direction. Lens 16 is part of the optical architecture intended to minimize a ROS defect commonly referred to as polygon wobble. After passing through the cylindrical lens 16 the laser beam is incident on a polygon 20 that includes a plurality of mirrored facets 22. The polygon is rotated at a constant rotational velocity by a motor (not shown) in a direction 24. The mirrored facets deflect the laser beam as the polygon rotates, resulting in a sweeping laser beam. A post-scan optical system 26 focuses the laser beam 10 to form a spot of circular or elliptic cross sectional shape on the photoreceptor 28. Significantly, the post-scan optical system 26 is typically an F-theta lens design intended to correct for scan line nonlinearity (see below). In FIG. 1, the direction of photoreceptor motion would be into (or out of) the view plane.
By properly modulating the laser beam 10 as the focused spot sweeps across the photoreceptor a desired latent image is produced That latent image is comprised of multiple scan lines, each of which is comprised of a plurality of image elements referred to as pixels. Ideally, the imaging system 6 should produce a geometrically straight scan line having evenly spaced, identically sized pixels. However, obtaining ideal pixels is difficult. The basic geometry of the flat image pane associated with an ordinary photographic lens design, such as found in a landscape or copy lens, leads to significant scan length distortion when used with a beam deflected at constant angular velocity. In such a lens, one degree of polygon rotation produces different lengths of deflection at different points along the scan path. The sweep distance per degree of rotation is greater near the ends of the photoreceptor 10 than at the center. As a consequence, the spot velocity varies as the spot scans across the photoreceptor, being higher towards the ends of the scan line and lower towards the center of the scan line. A varying spot velocity makes it necessary to adjust the modulation rate of the light source in order to compensate for the changes in spot velocity along the scan path and produce scan lines having evenly spaced, constant length, pixels. The source intensity may also need to be matched to the local spot velocity along the scan path in order to provide uniform exposure for discharging the photoreceptor. Without corrections the pixels would be further apart at the edges of the photoreceptor and closer together towards the center.
Various methods have been used to produce high quality scan lines of evenly spaced, uniformly sized pixels. In the post polygon optics imaging system 6 the lens 26 is typically of an F-theta design, wherein carefully engineered optical distortions have been incorporated in the optical design to compensate for the scan non-linearity as well as for polygon wobble. These lenses are referred to as F-theta designs because the focused spot position along the scan path is proportional to the product of the effective focal length F times the input deflection angle theta (in radians). Good F-theta designs have residual errors of only a few pixels. The result is a spot velocity that is a significant improvement over an uncorrected system, being relatively constant along the entire scan line. Unfortunately, the optical system of FIG. 1 typically employs bulky and expensive lens elements and requires some compromises in spot definition, image flatness, and color correction in order to allow reasonable distortion compensation in the design.
It is also possible to produce evenly spaced, uniformly sized pixels by tailoring the pixel clock frequency. Typically, the modulation rate is controlled using a pixel clock. If the pixel clock's frequency is properly varied, i.e., if the frequency matches the spot velocity at every point along the scan path, it is possible to exactly compensate for the varying spot velocity. However, in some critical applications it is important that the light source intensity also be matched to the local spot velocity along the scan path in order to compensate for the differences in spot dwell time and provide uniform exposure along the scan path. Otherwise pixels at the center of the scan line, where the spot velocity is lowest (for example) would be exposed more than those at the ends of the scan line. The extra exposure results in a slightly larger discharged area which causes spot growth, a process that is referred to as "blooming" or exposure smile error.
When variable frequency pixel clocks are used to produce scan lines having evenly spaced, uniformly sized pixels some means of synchronizing the pixel clock frequency with the spot position on the photoreceptor is required. FIG. 2 shows a prior art imaging system 30 that has an optical system very similar to that of the imaging system 6, except that the post polygon lens is not an F-Theta lens. The imaging system 30 includes a variable frequency pixel clock 31 and a start-of-scan detector 36. The variable frequency pixel clock produces a controlled frequency sweep that beings at a predetermined frequency upon the occurrence of a start-of-scan signal from the start-of-scan detector 36. The initial frequency and the frequency sweep profile are such that scan line pixels are properly spaced along the scan line. Start-of-scan detectors are well known. The one in the imaging system 30 incorporates a fiber-optic element 44 that guides light received at its input ends 46, which is in the tangential scan plane, to a photosensitive element (not shown). In response, the start-of-scan detector 36 produces the start of scan signal.
Prior art imaging systems have usually had variable frequency pixel clocks that are based upon phase locked loops (PLL). However, such systems have the disadvantages of complexity and expense. Furthermore, the phase locked loops that have been used employ analog feedback schemes that lack the noise immunity, stability and the reliability of a totally digital design. Therefore, a new technique of achieving a variable pixel clock frequency would be beneficial.
The following disclosures may relate to various aspects of the present invention.